The present invention disclosed herein relates to an external cavity laser, and more particularly, to a superluminescent diode (SLD) and a reflective semiconductor optical amplifier (R-SOA) that are used as external cavity laser light sources.
There are various light sources of an optical line terminal (OLT) and an optical network unit (ONU), which are used for a wavelength division multiplexing passive optical network (WDM-PON). In the previous paper (Optics Express, Vol. 16, No. 22, pp. 18194-18201), a planar lightwave circuit external cavity laser (PLC-ECL) is disclosed, which is fabricated by a hybrid integration method using an SLD and a polymer waveguide in which a grating are formed. The paper provides a detailed description of a wavelength tunable external cavity laser of which the wavelength can be varied by applying a current to a polymer grating. Owing to the use of the polymer grating, the wavelength tunable external cavity laser can be fabricated with low costs, and thus a WDM-PON using the wavelength tunable external cavity laser can be constructed with low costs.
Since characteristics of such an external cavity laser are generally determined by a light source, much research is being conducted on light sources. A light source of an external cavity laser should not lase, when a reflectivity of an output facet is less than 0.1%, and the light source should have high gain in a low operational current condition. Examples of light sources satisfying such requirements include a Fabry-Perot laser diode (FP-LD) and an SLD. An FP-LD includes an anti-reflection (AR) coated output facet having a reflectivity of less than 0.1% for reducing reflection at the output facet, and a high-reflection (HR) coated facet opposite to the output facet for increasing its gain. Although the output facet of an FP-LD has a reflectivity of less than 0.1% by anti-reflection coating, it is generally difficult to prevent lasing of the FP-LD, and the lasing yield is reduced. Nowadays, the Japanese NTT group produces multi-channel external cavity lasers by using anti-reflection and high-reflection coated FP-LDs. A light source suitable as such an anti-reflection FP-LD is an SLD.
Generally, an SLD includes an active layer or an optical waveguide sloped at an angle of 7°˜10° so as to reduce the reflectivity of an output facet. In this case, although the reflectivity of the output facet can be reduced, the SLD is not suitable for a WDM-PON due to an increased threshold current and operational current. Therefore, much research is currently underway to develop an SLD as a light source having characteristics comparable to those of an anti-reflection and high-reflection coated FP-LP.
An R-SOA is used in an ONU for the simple purpose of amplification and modulation of incoherent light assigned to each subscriber. Although there is a slight spectrum variation due to an external factor, the output power of an R-SOA is not largely varied, and owing to gain saturation, the R-SOA has low relative intensity noise. Therefore, much research is currently conducted to develop an R-SOA as a low-power-consumption light source having a low threshold current and operating current for using the R-SOA in a WDM-PON.